How the calculation works
This calculator uses the effective-impedance method with the AC resistance and reactance values in NEC Chapter 9, Table 9 (60 Hz, 75°C conductors) for PVC or aluminum conduit and for steel conduit. Effective impedance per 1,000 ft is Z = R·cosθ + X·sinθ, where cosθ is the load power factor (use 1.0 for resistive loads). Then VD = 2·I·L·Z ÷ 1000 for single-phase, or 1.732·I·L·Z ÷ 1000 for balanced three-phase, with L the one-way length in feet. NEC 210.19(A) and 215.2(A) informational notes recommend 3% on a branch circuit or feeder and 5% overall; CEC Rule 8-102 uses the same figures. Table 9 values are for three-phase 600 V cables and are used here as a close approximation for other voltages; large or paralleled conductors may need a full engineering analysis.
Worked example
20 A, 240 V single-phase, 100 ft one-way, 12 AWG copper in PVC conduit, power factor 1.0: Z = 2.0 Ω per 1,000 ft, so VD = 2 × 20 × 100 × 2.0 ÷ 1000 = 8.0 V (3.3%), which is a warning. 10 AWG (1.2 Ω per 1,000 ft) gives 4.8 V (2.0%) and passes.
Common mistakes to avoid
- Entering round-trip distance; the tool needs the one-way length and doubles it for single-phase.
- Treating 3% as a code limit; NEC lists 3% and 5% as recommendations in informational notes.
- Ignoring the temporary, larger drop during motor starting.
- Using copper values for an aluminum conductor.
Frequently asked questions
Is the 3% limit mandatory?
No. NEC lists 3% (branch) and 5% (total) as informational recommendations, but some jurisdictions, specs, or equipment may require them.
Is distance one-way?
Yes – enter the one-way run. The calculator doubles it automatically for single-phase.
Does conduit type affect the result?
Yes. Steel conduit raises reactance and, on large conductors, AC resistance compared with PVC or aluminum conduit. Pick the conduit type in the calculator and it applies the matching Table 9 values.
How is three-phase voltage drop different?
It uses 1.732 instead of 2 because the current is shared across three conductors in a balanced system.
Read the guide
Voltage Drop Explained: Formula, Limits and Examples
Related calculators
For informational and educational purposes only. Always consult a licensed electrician and local code authority. See our Electrical & Legal Disclaimer. Found an error? Tell us.